Cystine 103071 224992857 2008-07-11T10:36:36Z 66.123.206.90 Fix typo. {{NatOrganicBox | image= [[Image:Cystine-skeletal.png|thumb|right|[[Skeletal formula]] of cystine formed from L-cysteine (under biological conditions)]][[Image:Cystine-3D-balls.png|thumb|200px|right|[[Ball-and-stick model]] of cystine with the disulfide bond shown in yellow]] | name=Cystine | PubChem= | CAS = | SMILES = | C=6 | H=12 | N=2 | O=4 | S=2 | mass=240.30 }} '''Cystine''' is the [[amino acid]] [[dimer]] formed when a pair of [[cysteine]] molecules are joined by a [[disulfide]] bond. It is described by the formula (SCH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H)<sub>2</sub>. It is a colorless solid, and melts at 247-249 °C. It was discovered in 1810 by [[William Hyde Wollaston]] but was not recognized as a component of [[proteins]] until it was isolated from the [[Horn (anatomy)|horn]] of a [[cow]] in [[1899]].<ref> "cystine." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 27 July 2007 [http://www.britannica.com/eb/article-9028437 www.britannica.com/eb/article-9028437/cystine]</ref> Through formation of disulfide bonds within and between protein molecules, cystine is a significant determinant of the [[tertiary structure]] of most proteins. Disulfide bonding, along with hydrogen bonding and hydrophobic interactions is partially responsible for the formation of the gluten matrix in bread. Human hair contains approximately 5% cystine by mass.<ref>{{OrgSynth | author = Gortner, R. A.; W. F. Hoffman, W. F. | title = l-Cystine | collvol = 1 | collvolpages = 194 | year = 1941 | prep = CV1P0194}}</ref> ==Properties== The disulfide link is readily reduced to give the corresponding thiol, cysteine. This reaction is typically effected with thiols such as [[mercaptoethanol]] or [[dithiothreitol]]. :(SCH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H)<sub>2</sub> + 2 [[Thiol|RSH]] → 2 HSCH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H + RSSR == Nutritional sources == Supplemental [[N-acetyl cysteine]] is claimed to be a source of cystine, but the dose of this supplement is limited by side effects. One of the richest nutritional sources of cystine in the diet is [[Denaturation (biochemistry)|undenatured]] whey proteins from milk. The disulfide-bonded cystine is not digested or significantly hydrolized by the stomach, but is transported by the blood stream to the tissues of the body. Here, within the cells of the body, the weak disulfide bond is cleaved to give cysteine, from which [[glutathione]] can be synthesized. ====In animal feed==== Disulfide bonds can be broken at temperatures above about 150 [[Celsius|°C]], especially at low moisture levels (below about 20%)<ref>{{cite journal | author = M.A. Aslaksena, O.H. Romarheima, T. Storebakkena and A. Skrede | title = Evaluation of content and digestibility of disulfide bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources | journal = Animal Feed Science and Technology | volume = 128 | issue = 3-4 | pages = 320–330 | date = 28 June 2006 | doi = 10.1016/j.anifeedsci.2005.11.008 }}</ref>. == Side effects == Nutritional sources of cystine are virtually free of the toxic side effects associated with the single molecule of cysteine, N-acetyl cysteine. The greatest dietary source of cystine is bio-active, unpasteurized or low-heat pasteurized undenatured whey proteins.{{Fact|date=June 2007}} ==See also== * [[Cystinuria]] * [[Cysteine]] == References == <references /> ==External links== {{E number infobox 920-929}} [[Category:Organic disulfides]] [[Category:Sulfur amino acids]] [[ar:سيستين]] [[de:Cystin]] [[es:Cistina]] [[fr:Cystine]] [[it:Cistina]] [[nl:Cystine]] [[ja:シスチン]] [[pl:Cystyna]] [[pt:Cistina]]